1. Field of the Invention
The present invention relates to a stage apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
An exposure apparatus is employed in a photolithography process for fabricating, for example, a semiconductor device. The exposure apparatus includes a projection exposure apparatus which projects and transfers the pattern of a reticle (mask) onto a wafer (substrate) by a projection optical system, and an electron beam exposure apparatus which directly draws a pattern on a wafer without using a reticle.
With miniaturization of semiconductor devices, such an exposure apparatus must improve the overlay accuracy of a pattern to be transferred onto the wafer to form a multilayered structure, and is therefore required to attain a pattern transfer position accuracy on the order of several nanometers or less. Hence, the exposure apparatus executes, before an exposure process, an alignment process, that is, global alignment as an example. In the global alignment, alignment marks formed in several representative shot regions on the wafer are measured, and the measurement results are statistically processed, thereby obtaining the exposure target position of each shot region on the wafer.
On the other hand, in the exposure apparatus, due, for example, to vibration, an external force, or a change in temperature, deformation is often generated in a table which holds the wafer or a base portion which supports an interferometer for measuring the position of the table. Thermal deformation is generated in the table or wafer when heat produced by a driving unit which drives the table, for example, is transmitted to the table, or a change in temperature is generated in the wafer due to exposure heat (for example, the energy of an electron beam). Also, the base portion vibrates when vibration generated upon driving the table, or disturbance vibration of the floor on which a stage apparatus is installed, is transmitted to it. The vibration of the base portion includes rigid vibration and elastic vibration that mainly constitute the entire vibration of the base portion, and three-dimensional bending deformation is generated in the base portion due to the elastic vibration. Thermal deformation is generated in the base portion as well when heat produced by the driving unit which drives the table, for example, is transmitted to it. If deformation is generated in the base portion during the alignment process, the position of the interferometer for measuring the position of the table fluctuates, so an error is generated in the measurement value obtained by the interferometer, and an error, in turn, is generated in the obtained exposure target position. In contrast, if deformation is generated in the table or base portion after the alignment process, a transfer error such as a shift in position or a shift in magnification (a shift in size) is generated in the transferred pattern even when the wafer (the table which holds it) is positioned at the exposure target position obtained by the global alignment.
Several techniques of correcting the exposure target position to compensate for the deformation of the table or base portion have conventionally been proposed. Japanese Patent Laid-Open No. 2005-317600 discloses a technique of arranging two interferometers on opposite sides of the table, and obtaining the deformation amount of the table from the measurement values (that is, the positions of the two end surfaces of the table in one direction) obtained by the two interferometers, thereby correcting the exposure target position based on the obtained deformation amount. Also, Japanese Patent Laid-Open No. 11-008189 discloses a technique of obtaining, in advance, a correction equation expressing the relationship between the measurement error generated by the interferometer and the value (the deformation amount of the base portion) output from a strain gauge disposed on the base portion which supports the interferometer, thereby correcting the exposure target position using the correction equation and the value output from the strain gauge in the exposure process.
Unfortunately, in the technique disclosed in Japanese Patent Laid-Open No. 2005-317600, if deformation (for example, thermal deformation) is generated in the base portion which supports the two interferometers, the deformation amount of the base portion is included in that of the table, which is obtained from the measurement values obtained by the two interferometers, thus making it impossible to correct the exposure target position with high accuracy. This is because the measurement value obtained by each of the two interferometers contains three components: the position of the table, the deformation amount of the table, and the deformation amount of the base portion (that is, a fluctuation in distance between the two interferometers).
Assuming, for example, that the linear expansion coefficient of the base portion is 12×10−6 [1/° C.], the distance between the two interferometers is 1 [m], and a change in temperature of 0.001 [° C.] is generated in the base portion, the distance between the two interferometers considerably fluctuates by 12 [nm]. At this time, the deformation amount of the table, which is obtained from the measurement values obtained by the two interferometers, includes an error of 12 [nm] corresponding to the deformation amount of the base portion, thus making it difficult to correct the exposure target position on the order of several nanometers or less. Note that the distance between the two interferometers is assumed to be 1 [m] because this numerical value is appropriate for (the stage apparatus of) an exposure apparatus which exposes a wafer having a diameter of 300 [mm]. More specifically, the length of each side of the table which holds the wafer is about 500 to 600 [mm], and the range in which the table can move is about ±200 to ±250 [mm], so the two interferometers must have a distance of about 1 [m] between them. Also, the base portion has a relatively high volume and, in turn, a relatively high heat capacity, so a change in temperature is less likely to be generated in the base portion, but nonetheless only a minute change in temperature of about 0.001 [° C.] is generated in the base portion due to factors associated with the surrounding environment or temperature regulation accuracy.
On the other hand, in the technique disclosed in Japanese Patent Laid-Open No. 11-008189, the value output from the strain gauge and the measurement error generated by the interferometer (that is, the fluctuation in position of the interferometer) do not always have a unique correspondence. This is because two types of deformation, expansion/contraction deformation mainly due to thermal deformation, and three-dimensional bending deformation mainly due to vibration (in the Z-axis direction), are generated in the base portion.
FIG. 8A is a view showing a fluctuation in position of the interferometer when expansion/contraction deformation is generated in the base portion, and FIG. 8B is a view showing a fluctuation in position of the interferometer when bending deformation is generated in the base portion. Note that referring to FIGS. 8A and 8B, dotted lines indicate the state before deformation is generated in the base portion, and solid lines indicate the state after deformation is generated in the base portion. As can be seen from FIGS. 8A and 8B, even when a strain gauge is disposed, for example, near the interferometer on the base portion, the coefficient for converting the value output from the strain gauge into a fluctuation in position of the interferometer changes depending on whether expansion/contraction deformation or bending deformation is generated. Therefore, the technique disclosed in Japanese Patent Laid-Open No. 11-008189 often cannot correct the exposure target position with high accuracy.
Supposedly, it is possible to arrange a plurality of strain gauges on the base portion to divide the entire deformation of the base portion into expansion/contraction deformation and bending deformation from the values output from the plurality of strain gauges. However, in this case, there arise problems that, for example, the apparatus arrangement becomes more complex upon arranging a plurality of strain gauges, and a correction equation must be obtained for each type of deformation (expansion/contraction deformation and bending deformation) generated in the base portion (that is, it takes much time to correct the exposure target position). Furthermore, to obtain correction equations, expansion/contraction deformation and bending deformation must independently be generated in the base portion, but this is difficult in practice, thus making it hard to precisely obtain a correction equation for each type of deformation. When, for example, a correction equation for expansion/contraction deformation is to be obtained, it is difficult to completely cut off vibration that generates bending deformation in the base portion. Similarly, when a correction equation for bending deformation is to be obtained, it is difficult to completely eliminate thermal deformation (a change in temperature) that generates expansion/contraction deformation in the base portion. Therefore, even when the exposure target position is corrected using correction equations obtained in advance, it includes an error to a certain extent. Also, processing of dividing the entire deformation of the base portion into expansion/contraction deformation and bending deformation from the values output from the plurality of strain gauges, and processing of selecting an appropriate correction equation, for example, are necessary in the alignment process, and this may lower the apparatus throughput.